Channels failed to be subscribed to cannot be unsubscribed from

[yu840915]

Steps to reproduce

1. Create a Pusher client with custom authorizer, accessing external authorization endpoint and having chance to fail, e.g. network failure.
2. Subscribe to a channel with airplane mode on.
3. Unsubscribe from the channel.

Expected behavior

Swift SDK allows unsubscribing, clears the channel, like Java SDK.
Source

Actual behavior

The channel is stuck in PusherChannels.channels.

We found a guard clause may be related to the issue.
Source

The stuck channels can be unexpectedly subscribed to later when the app comes back from background and reconnects when we think they are dead.

Any improvements you suggest

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